The region of arterial branches is known to be an early site of the origin of atherosclerosis. Two of the possible mechanisms for this localization relate to hydrodynamic effects. Shear stresses may damage endothelium where flow dividers exist, and wall oxygenation may be impeded where incipient separate flow occurs, as shown in our previous analyses. Three dimensional branch regions are difficult to examine mathematcally, experimentally, and relatively little data is available for flow field and oxygen transport definition. We propose an approach combining mathematical and physical modeling with in vivo wall oxygenation measurements. The Navier-Stokes equations and oxygen transport equation will be solved numerically in order to acquire a quantitative understanding of the interactions between branch flow patterns, oxygen transport to the wall, and early atherosclerosis. Hollow vascular replicas made directly from previously studied animal branch structures will be used in flow studies to provide feedback to the computer analyses. In vivo experiments in rabbits and dogs will provide input flow and pressure data. In these same animals, arterial walls at branch junction areas will be explored systematically using the oxygen sensitive microcathode to test predictions, provide feedback for mathematical modelling, and to test the hypothesis that flow structure determined hypoxia is a factor in early appearance of atherosclerosis.